| Summary: | In the southeast of European Russia, the gall wasp Aulacidea hieracii (Bouché, 1834) is attacked by ten parasitoid species, including Eupelmus (Eupelmus) microzonus Förster, 1860 and E. (Macroneura) messene Walker, 1839. Although both members of the genus Eupelmus Dalman, 1820 are idiobiont ectoparasitoids, they demonstrate different life-history strategies in respect to many bionomic features. Specifically, E. messene is represented by brachypterous thelytokous females which lay single eggs directly onto the host body. This species can parasitize both concealed and exposed larvae and pupae of A. hieracii, but fails to attack its primary parasitoids. On the contrary, arrhenotokous males and females of E. microzonus are fully winged. These parasitoids usually lay several eggs per host which are placed onto the wall of the host chamber and covered with a particular fibrous substance. E. microzonus never parasitizes pupae or exposed larvae, although it can readily attack concealed larvae of A. hieracii and its primary parasitoids. In addition, hibernating individuals of E. messene undergo obligatory larval diapause, but those of E. microzonus are able to develop without exposure to subzero temperatures. All these data collectively suggest that the former species is highly specialized to exploit A. hieracii as a host, whereas the latter one mostly exhibits the so-called morphotypical specialization. These different strategies allow E. messene and E. microzonus to coexist on the same host species, as a local specialist and a more or less evenly distributed generalist, respectively.
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